In optical apparatuses such as digital still cameras, as the number of pixels and the magnification of zooming increase, the blurring of image due to hand vibration (including tremble of hands in addition to the sway of arms or a body) while photographing increases. To prevent it, the optical apparatuses have become comprising anti-vibration devices. The anti-vibration device is an apparatus for moving image to the right position on an image sensor such as CCD, etc. depending on hand movement detected while photographing.
The anti-vibration device mainly uses an image-sensor-shifting system (for example, CCD-shifting system) or a lens-shifting system. The image-sensor-shifting system is a system of shifting an image sensor (CCD) based on hand movement detected by a vibration-detecting sensor such as a gyro sensor, etc. to correct image blur. The lens-shifting system is a system of shifting a correction lens contained in a barrel comprising imaging lenses vertically and horizontally in a plane perpendicular to an optical axis of the imaging lenses by a very small distance (several millimeters or less) determined by vibration detected by a vibration-detecting sensor, to change the refraction of light, thereby correcting image blur. The terms “hand vibration” and “image blur” are sometimes used for the same meaning, but only the term “hand vibration” is used herein to avoid confusion.
The correction of hand vibration by the image-sensor-shifting system, which need not shift the optical system, is more advantageous than the lens-shifting system for miniaturization, so that it is mainly used in compact-sized (portable) digital cameras. On the other hand, the correction of hand vibration by the lens-shifting system is excellent in the function of correcting hand vibration but disadvantageous in miniaturization, so that it is mainly used in interchangeable lens units for digital, single-lens reflex cameras and digital video cameras. Although the lens-shifting system can theoretically completely correct hand movement, it needs the high-precision shifting of a correction lens in a plane perpendicular to the optical axis of the imaging lenses.
Electric power consumption by a digital camera, etc. is not constant but increases temporarily when photographing (releasing a shutter), when storing data in a memory card, etc., followed by drastic voltage drop, so that the voltage of a battery may be lower than that of operating the cameras. Accordingly, it is preferable to avoid increase in current supplied to a correction-lens-shifting means (for example, a voice coil motor) for the correction of hand movement.
The anti-vibration device by the lens-shifting system comprises a voice coil motor (VCM) as a correction-lens-shifting means, which comprises magnetic circuits comprising permanent magnets and yokes, and movable coils fixed to a frame holding a correction lens. To improve shifting efficiency without making the VCM larger, Japanese Patent 4,181,663 discloses an anti-vibration device comprising, as shown in FIG. 9(a), permanent magnets 13c, 13d polarized to have a pair of magnetic poles via unmagnetized neutral zones 13f1, 13f2, and a flat coreless coil 12 moving in a magnetic gap between the permanent magnets 13c and 13d, the effective conductor width Wb of the coreless coil 12 and the magnetic pole width Wm of the permanent magnets 13c, 13d meeting the relation of Wb<Wm≦Wb+Sc, wherein Sc represents the maximum movement distance in one direction of the coreless coil 12, which corresponds to ½ of St shown in FIG. 9(a). In addition to this relation of size, this VCM has such a structure that the width of a coreless portion of the coreless coil 12, the width of the neutral zones 13f1, 13f2 of the permanent magnets 13c, 13d, and the maximum one-side movement distance Sc of the coreless coil 12 are the same, to obtain the maximum thrust when the correction lens moves by the maximum distance Sc. Because this VCM has the unmagnetized neutral zones 13f1, 13f2 between the opposing permanent magnets 13c, 13d, a volume percentage of the permanent magnet contributing to the generation of magnetic flux in the magnetic gap is smaller, resulting in a smaller amount of magnetic flux, meaning poorer magnetic efficiency. As a result, to obtain the predetermined thrust, large current should be supplied to the coil.
Further, although the VCM described in Japanese Patent 4,181,663 has slightly improved moving efficiency, a portion 51 of the coil 12 goes out of the magnetic gap (comes out of opposition to the permanent magnets 13c2, 13d2) when the coil moves from a center position (the original position) P0 by distance Sc to one direction as shown in FIG. 9(b), resulting in drastic decrease in thrust in both end regions of the movement range of the coil. Accordingly, when the coil 12 moves, for example, in a direction F1 in FIG. 9(b), large current should be supplied to the coil to compensate thrust drop in both end regions S1 of the movement range. Thus, in the anti-vibration device comprising the VCM described in Japanese Patent 4,181,663, electric power consumption increases at the time of correcting hand movement, because a portion 51 of the coil 12 goes out of the magnetic gap due to the neutral zones 13f1, 13f2 between the permanent magnets 13c, 13d. 
An anti-vibration device described in Japanese Patent 2,641,172 comprises VCM, which comprises a magnetic circuit constituted by a pair of permanent magnets arranged with a predetermined gap and a yoke forming the magnetic gap, and a coil moving inside the magnetic gap. Like the unmagnetized neutral zones in Japanese Patent 4,181,663, the magnetic gap between a pair of permanent magnets is undesirable for magnetic efficiency, because it has a magnetic flux density reduced correspondingly to the volume decrease of the permanent magnets as compared with the gap-free structure. To compensate the decreased magnetic flux density, larger current should be supplied to the coil. In addition, Japanese Patent 2,641,172 does not describe the relations among the effective conductor width of the coil, the magnetic pole width of each permanent magnet and the longest one-direction movement distance of the coil, failing to take into consideration decrease in electric power consumption at the time of correcting hand movement.
An anti-vibration device described in JP 2008-209434 A comprises VCM, which comprises a pair of opposing yokes, a permanent magnet fixed to one yoke, and a coil moving inside a magnetic gap defined by the other yoke and the permanent magnet. The permanent magnet is magnetized to have two magnetic poles, N and S poles, in a plane. Such permanent magnet is substantially the same as those described in Japanese Patent 4,181,663 having the unmagnetized neutral zones between an N pole and an S pole. Likewise, JP 2008-209434 A does not describe the relations among the effective conductor width of the coil, the magnetic pole width of each permanent magnet, and the longest one-direction movement distance of the coil, failing to take into consideration decrease in electric power consumption at the time of correcting hand movement.